Methane—Methane gas is present in
the atmosphere as the result of natural biological processes like the anaerobic decomposition of organic material in rice fields. This decomposition is called methanogenesis, and more than 100 million tons of rice production annually produces significant amounts of the gas, most of which is emitted into the atmosphere. Other sources of the gas include cattle ranching, coal mining and increased use of natural gas.
Direct atmospheric measurement of atmospheric methane has been possible since the late 1970s and its concentration rose from 1.52 ppmv (parts per million by volume) in 1978 by around 1 percent per year to 1990 when its concentration leveled off at 1.77 ppmv. The concentration of methane in the atmosphere is less than 1% of the carbon dioxide concentrations, but methane is 25 times more effective at absorbing infrared radiation so it should be considered an important factor in global warming. Estimates indicate that it accounts for about 10% of the greenhouse effect.
Nitrous Oxide—Increased use of fertilizer is the main cause for the increase in N2O concentration in the atmosphere since the beginning of the Industrial Revolution. Concentration for N2O in 1998 was 314 ppb, making it responsible for 6% of the greenhouse effect. In addition to agricultural sources, some industrial processes like fossil fuel-fired power plants, nylon production, nitric acid production and vehicle emissions are also sources. Nitrous oxide is 230 times better at absorbing IR radiation than CO2.
CFCs—The only major greenhouse gases that are not naturally occurring are the chlorofluorocarbons. They come from sources such as the production and/or use of foams, aerosols, refrigerants, and solvents. They are present at an extremely low concentration in the atmosphere, but they are 15,000 times more efficient as a greenhouse gas relative to carbon dioxide. As a result they contribute to approximately 25% of the total greenhouse effect based on 1990 concentrations. They were first produced in large quantities in the late 1920s and, since then, concentrations of CFCs in the atmosphere have been rising. Due to the discovery that they are able to destroy stratospheric ozone, major international initiatives are under way to restrict their production and use. However, their long atmospheric lifetimes mean that some concentration of the CFCs will remain in the atmosphere for over 100 years.
Other Gases—There are three other gases that are related—directly or indirectly—to the greenhouse effect. They are ozone, carbon monoxide and volatile organic compounds. Even though increased ozone concentrations appear primarily in the Northern Hemisphere, near cities, and are cyclic in nature, ozone is considered by the International Panel on Climate Change to be an important greenhouse gas. Ozone concentrations have risen by 30% since the Industrial Revolution. Carbon monoxide (CO) is not considered a greenhouse gas, but it affects the production of both methane and ozone, themselves greenhouse gases. In addition, VOCs are also instrumental in the production of ozone.
The table below summarizes the major greenhouse characteristics, sources and concentrations of greenhouse gases.